It is well known in the art to build a form of wood, metal or other sheet material to act as a mold into which concrete is poured and then allowed to set. It is also well known that the strength of the resulting concrete slab may be greatly increased by pouring the concrete around reinforcing steel, known as rebar, that may be positioned within the form prior to adding the concrete.
For best results and maximum strength, the concrete surrounds and fully encases the reinforcing steel. Where heavy reinforcing steel is utilized, the steel should be supported above the bottom wall or floor of the form. This can be accomplished by utilizing bolsters.
Bolsters are relatively lightweight frame members that are positioned at spaced intervals on the floor of the form. Each bolster includes a rebar or beam supporting surface that is spaced from the form floor. After placing the necessary bolsters in position within the form, the rebar is positioned extending between and across the support surfaces of the bolsters. The concrete that is then poured into the form can flow around the rebar even into the space between the rebar and the form floor. Thus, the rebar is fully surrounded and encased by the concrete as desired to provide maximum strength to the concrete slab.
For many years, the road construction industry has used bolsters constructed from metal wire stock. Such bolsters are relatively inexpensive to produce. They also provide sufficient strength to support reinforcing steel in proper position during the pouring and setting of concrete. Recent studies have, however, identified a significant shortcoming to their use. More specifically, metal wire bolsters promote spalling of the concrete.
In spalling, pieces of concrete and in some instances, large chunks break away from the main concrete slab. This is a particularly dangerous safety condition on, for example, roadway bridges. Spalling not only reduces the integrity of the structure but if the condition goes unchecked, it can even lead to a bridge collapse. There, of course, is also the added danger that pieces of concrete falling from a bridge structure could land on vehicles or individuals passing under the bridge.
Research has shown that the causes of spalling are water related. Once the concrete slab sets, the form is removed leaving the legs of the bolster that previously rested upon the bottom wall of the form exposed. Air and moisture seeping around and under the slab cause the exposed legs of a metal wire bolster to corrode and rust over time. Moisture also has a tendency to be drawn upwardly into the slab by capillary action along the interface between the metal bolster and the concrete In fact, the moisture may even be drawn deep into the formation along the rebar. As the bolster and the rebar rust, gas is released. Eventually, sufficient gaseous pressure may build up within the slab so as to cause pieces of concrete to actually be exploded from the main slab.
The spalling action may be further accelerated and accentuated by low temperatures. More specifically, subfreezing temperatures cause water trapped within the concrete slab in the area of the bolster and rebar interfaces with the concrete to freeze. As the water freezes, it expands exerting tremendous forces on the concrete that can cause pieces of the concrete to break away from the slab.
Several different approaches have been utilized to date to address and overcome the spalling problem. Bolsters constructed from galvanized wire and even stainless steel are available. While these do substantially reduce the problems of spalling from the rusting of a bolster, they do not represent a fully acceptable solution for a number of reasons. One is that they are relatively expensive to produce. Another is that they still do not bond well with the concrete. Thus, water can still be drawn by capillary action deep into the formation along the interface between the bolster and the concrete. This water can move up along the bolster to the rebar and thus still cause rusting problems that lead to spalling. The water, of course, can also freeze in the formation and thereby cause spalling in this manner as well.
Another alternative approach has been to utilize bolsters constructed of metal wire stock dipped in and coated with epoxy. While the epoxy coating does prevent the legs of the bolsters from rusting, the coating is, unfortunately, prone to chipping either during simple handling of the bolsters or during removal of the form after the concrete sets. Rusting of the resulting exposed metal surface is, of course, likely. In addition, the epoxy coating does not bond with the concrete. Thus, water can still enter by capillary action into the formation and cause spalling as discussed above. A need is, therefore, identified for a new and improved bolster.